In order to remove substances usually eliminated with urine and for fluid withdrawal, various methods for extracorporeal blood treatment or cleaning are used in chronic kidney failure. In haemodialysis, the patient's blood is cleaned outside the body in a dialyser. The dialyser has a blood chamber and a dialysis fluid chamber, which are separated by a semipermeable membrane. During the treatment, the patient's blood flows through the blood chamber. In order to clean the blood effectively from substances usually eliminated with urine, fresh dialysis fluid flows continuously through the dialysis fluid chamber.
Whereas the transport of the low-molecular substances through the membrane is essentially determined by the concentration differences (diffusion) between the dialysis fluid and the blood in haemodialysis (HD), substances dissolved in the plasma water, in particular higher-molecular substances, are effectively removed in haemofiltration (HF) by a high fluid flow (convection) through the membrane of the dialyser. In haemofiltration, the dialyser functions as a filter. Haemodiafiltration (HDF) is a combination of the two processes.
In haemo(dia)filtration, part of the serum drawn off through the membrane is replaced by a sterile substitution fluid, which is fed to the extracorporeal blood circuit upstream of the dialyser (pre-dilution) or downstream of the dialyser (post-dilution).
Devices for haemo(dia)filtration are known in which the dialysis fluid is produced online from fresh water and concentrates and the substitution fluid is produced online from the dialysis fluid.
In known haemo(dia)filtration devices, the substitution fluid is fed to the extracorporeal blood circuit from the fluid system of the machine via a substitution fluid line. In pre-dilution, the substitution fluid line leads to a connection point on the arterial blood line upstream of the dialyser, whilst in post-dilution the substitution fluid line leads to a connection point on the venous blood line downstream of the dialyser. The substitution fluid line has a connector, with which it can be connected either to the venous or arterial blood line. In order to interrupt the fluid supply, a clamp or suchlike is provided on the substitution fluid line.
The correct connection of the substitution fluid line is routinely checked before the commencement of the blood treatment with known haemo(dia)filtration equipment. For this purpose, the line leading to the dialysis fluid chamber and leading away from the dialysis fluid chamber of the dialyser and the venous blood line downstream of the connection point for the substitution fluid line are clamped by means of tube clamps. The arterial blood line is already interrupted by the stationary blood pump upstream of the connection point for the substitution fluid line. The substituate pump for conveying the substitution fluid is then started, and the pressure in the venous blood line is measured by means of a venous pressure sensor.
In the event that a pressure in the venous blood line cannot be built up with the substituate pump that is greater than a preset limiting value, the conclusion is drawn that the connection of the substitution fluid line is not correct, i.e. the supply of fluid is interrupted. During the blood treatment, it can happen in dialysis practice that the treatment procedure is switched between post- and predilution. For this purpose, the clamp is closed on the substitution fluid line, and the substitution fluid line is separated from the venous or arterial blood line and connected to the arterial or venous blood line respectively. It cannot be ruled out in practice that the opening of the tube clamp may be forgotten. If the substituate pump is not stopped and the pressure test described above is not carried out, this state is not detected. It is a drawback that, with the known haemo(dia)filtration equipment, the supply of substitution fluid is not monitored during the treatment. An interruption of the substituate supply, therefore, remains undetected. In this case, haemodialysis can be carried out with only little effect during HDF treatment. During HF treatment, the patient is then not treated at all in the extreme case. This can have more or less serious consequences for the patient, although he/she is not directly endangered.